serial: tegra: Correct shutdown of UARTs

There are two issues in the shutdown path of the UARTs which are:
1. The function tegra_uart_shutdown() calls tegra_uart_flush_buffer()
   to stop DMA TX transfers. However, tegra_uart_flush_buffer() is
   called after the DMA channels have already been freed and so actually
   does nothing.
2. The function that frees the DMA channels
   (tegra_uart_dma_channel_free()), unmaps the dma buffer before
   freeing the DMA channel and does not ensure the DMA has been
   stopped.

Resolve this by fixing the code in tegra_uart_dma_channel_free() to
ensure the DMA is stopped, free the DMA channel and then unmap the DMA
buffer. Finally, remove the unnecessary call to tegra_uart_flush_buffer().

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>